Immune Checkpoint Blockade Promotes Adult T Cell Leukemia Abstract Human T-cell leukemia virus type 1 (HTLV-1) is the cause of a T cell lymphoproliferative disorder, designated adult T-cell leukemia lymphoma (ATL). This is a highly refractory malignancy, lacking effective treatment approaches, with a long-term survival rate of less than four percent. The current project is based on our clinical trial in ATL patients of immune checkpoint blockade with PD-1 antibody, nivolumab. Although our trial was designed to enroll up to 20 ATL patients, all of the first 3 patients exhibited marked, rapid acceleration of their disease after a single dose of nivolumab. Thus, these individuals were taken off the clinical trial, given salvage chemotherapy, and accrual to the trial was discontinued. The current project is designed to gain a mechanistic basis for this unexpected and paradoxical effect, in order to better define the benefits and risks of immune checkpoint blockade, especially when it is considered for T-cell malignancies. For this purpose, we will use baseline and progression clinical samples from the 3 subjects who participated in this clinical trial. In addition, we have cryopreserved ATL samples from 37 other subjects who participated in our previous multicenter clinical trials. Our aims in this study are as follows: Aim 1. Define the effect of immune checkpoint blockade on tumor cell clonality. 1a. Genomics, 1b. Transcriptomics. We will determine if immune checkpoint blockade selects for specific ATL subclones in culture, in PDX models in mice, and in patients, by analysis of variant allele frequencies of cellular gene mutations, T cell receptor (TCR) V? repertoire, and viral integration sites. We will assess the ATL transcriptome to determine if CD28 and/or TCR signaling is uncoupled by immune checkpoint blockade, leading to increased PI3K/ATK, TCR, and/or NF?B signaling. Aim 2. Define the effects of immune checkpoint blockade on T cell signaling pathways: a) Proliferation rate, b) PD-1, PD-L1 levels, c) PI3K and RAS/MAPK pathways. We will assess the effects of immune checkpoint blockade in culture, in PDX models in mice, and in patients by FACS to assess proliferation, apoptosis, AKT and ERK activation. Aim 3. Define the effect of immune checkpoint blockade on T cell proliferation: a) In culture and b) In NSG mice. In addition to the studies above, we will use primary ATL cells, ATL cell lines, and PDX models to assess signaling events through immunoblot studies of the PI3K/AKT and NF?B pathways. Understanding the molecular mechanism for malignant T-cell proliferation after immune checkpoint blockade is important to provide a rational understanding of the benefits and adverse effects of this therapy.